I'm going through this decision right now. I agree, you are building a product with an unproven market and lots of time to grow organically, maybe you do want to start small and scrappy. Build something you can easily throw away and start over with. Build something that gets you to market as quickly as possible so you can pivot.

OTOH, If you are trying to sell the idea to investors and large companies that you are a serious player and have a plan and know-how to grow and scale your service quickly, maybe you do want to show that you have the design chops and ability to actually scale your product. Take a look and ask yourself, "Does my business model only work if it scales up dramatically, far beyond the capacity of a single database?" If the answer is "yes", start with a scalable architecture to save the 100+ person-years and endless gnashing of teeth it will take to untangle your monolith (been there.)